Nope, the game is running in 25 fps, nothing to do with youtube. 50 fps scroll would be twice as fast as the one in the game. If you save the video in the emulator you will see that each two frames are the same.

The coder made it to run on 50fps on real hardware. You're wrong here.

Emulators are not replicating perfectly the CRTC emulation and timing. That why they also ask to play R-type 2012 on a real CPC and not an emulator.